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| Proteins |
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| Proteins are the chief body builders of the body. They are complex molecules made up of carbon, hydrogen, oxygen and nitrogen (sometimes sulphur and phosphorus). Proteins are used to synthesize enzymes (e.g., pepsin, trypsin), hormones (e.g., insulin, adrenaline), carrier proteins (e.g., Haemoglobin), contractile proteins (e.g., myosin, actin), structural proteins (e.g., collagen) and protective proteins (antibodies). They also form skin pigments like melanin and nucleic acids of the genetic material, DNA and RNA - purines and pyrimidines. |
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| Normal protein requirement of an adult is 1g/kg of the body weight. However, growing children, pregnant and nursing mothers need more proteins. |
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| Sources of Proteins are of both animal and plant origin. Some of the common sources are groundnuts, beans, whole cereals, pulses. among plants and fish, egg, meat, milk, cheese among animal products. |
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| Composition of proteins |
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| The proteins are polymers made of monomers called the amino acids. There are 20 different kinds of amino acids that make up the proteins. However, they are present in different proportions in each of the proteins. |
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| Classification of amino acids |
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| There are two types of amino acids as per the nutritional requirement. They are classified into essential and non-essential amino acids. |
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| Essential amino acids are those that are not synthesized by the body and are needed in the diet. Methionine, threonine, tryptophan, valine, isoleucine, leucine and phenylalanine are the essential amino acids. |
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| Non-essential amino acids are those that are synthesized in the body and are not necessary in the diet. Glycine, alanine, serine, cysteine, tyrosine, proline, aspartic acid and glutamic acid are the non-essential amino acids. |
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| Note: |
| Semi-indispensible amino acids are arginine and histidine. |
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| Most plant proteins lack in one or more essential amino acids and thus should be taken in combination to fulfill all the protein requirements. On the other hand, animal proteins contain all essential amino acids. Thus they are better providers of complete protein requirements. |
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| Proteins are classified based on their composition. They are of two types: |
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| Simple proteins |
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| Simple proteins are those which contain only amino acids. |
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| Example: Egg albumin and seed globulin. |
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| Conjugate proteins |
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| Conjugate proteins are those which contain a non-amino acid component in addition to the amino acids. This non-amino acid component is called the prosthetic group. For e.g., glycoprotein (with carbohydrate as the prosthetic group), phosphoprotein (with phosphate as the prosthetic group), lipoprotein (with lipid as the prosthetic group). |
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| Functions of proteins |
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| 1) Proteins build new tissues of the body. |
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| 2) They maintain and replace damaged tissues. |
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| 3) They carry out regulating activities as enzymes and hormones. |
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| 4) They are protective as antibodies. |
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| 5) They help in other important activities such as movement of skeletal muscles, transport of oxygen, pigmentation of skin, etc. |
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